The volumetric expansion of tobacco material, such as cut filler, to increase its filling capacity is well-known in the art of tobacco processing. One method for the volumetric expansion of tobacco material involves impregnation of the tobacco material with liquid carbon dioxide (CO.sub.2), subjecting the CO.sub.2 impregnated tobacco material to conditions sufficient to convert substantially all of the liquid CO.sub.2 to solid CO.sub.2, then vaporizing the solid CO.sub.2 in the impregnated tobacco material so as to expand the tobacco. This process has been referred to in the art as a dry ice expanded tobacco process or "DIET" process. An example of the DIET process is disclosed in U.S. Pat. No. 5,259,403 assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference.
The DIET process is typically practiced by introducing particles or "clumps" of solid CO.sub.2 impregnated tobacco material into a heated gas stream which is accelerated by a venturi. The heated gas conveys the tobacco material through a duct and sublimates or volatilizes the solid CO.sub.2 to cause expansion of the tobacco material. The conveying duct, sometimes referred to as a sublimator, is usually in the form of a vertical or upwardly inclined tube or pipe with a cylindrical or rectangular cross-section. The particles or clumps of impregnated tobacco material are entrained in the sublimator tube until the solid CO.sub.2 is substantially completely sublimed or volatilized. From the sublimator, the expanded tobacco material is transported to a separator apparatus, such as a tangential separator, cyclone separator or the like, where it is separated from the hot gas stream, tobacco volatiles and dust.
According to the apparatus disclosed in the aforesaid U.S. Pat. No. 5,259,403, the conveying duct or sublimator is in the form of a vertically extending duct having a circular cross-section that increases from a smaller diameter at the inlet thereof to a larger diameter at an intermediate portion thereof. Advantageously, that construction provides a reduced velocity section of the sublimator which prevents transport of large clumps of solid CO.sub.2 impregnated and unexpanded tobacco material into the tangential separator.
Other conventional sublimator apparatuses for practicing the DIET process have a number of limitations or deficiencies. For example, in many sublimators, the inlet valve or air lock for introducing the clumps of solid CO.sub.2 impregnated tobacco material into the duct often admits excessively large incremental quantities of material into the heated gas stream at the duct inlet at a relatively slow rate which results in a nonuniform distribution of tobacco material in the sublimator. Poor scattering and lack of entrainment of the impregnated tobacco particles and clumps upon entering the heated gas stream and sublimator result in variable dwell times and variations in the amount of heating and expansion of the tobacco particles. As a result, some particles are darkened and burnt by overheating and others are light and only partially expanded. This is especially problematic with large clumps of tobacco material which tend to fall to the bottom of the duct where there is poor air flow and poor heat exchange in the prior art apparatuses.
The use of 90.degree. elbows and other angled duct sections to minimize the floor area of a plant taken up by a DIET apparatus results in excessively non-uniform heated gas flows through the duct and greater breakage of the tobacco particles because of the abrupt direction changes at the elbows and through the use of impingement plates. Non-uniform gas flows result in "jetting" or "roping," i.e., one region flowing at a greater velocity than another, causing significant dwell time variations and uneven heating. Excessive gas flow velocity also causes breakage of tobacco strands. Some duct designs experience significant gas recirculation zones which also adversely affect dwell time of the tobacco material in the sublimator.
To achieve maximum filling capacity or filling power of the expanded tobacco product of the sublimator, the solid CO.sub.2 impregnated tobacco material must be expanded to the greatest extent possible without overheating or excessive breakage of the tobacco strands. It would be desirable therefore to provide a sublimator apparatus and a method of expanding tobacco to a maximum filling capacity with no overheating and minimum breakage of the tobacco strands while maximizing the tobacco throughput of the apparatus.